This invention relates to AM radio receivers having a noise blanking circuit and more particularly to such a receiver wherein the blanking circuit responds to a noise spike by blanking both the modulated-AM-signal path and the audio-signal path.
This invention relates to all kinds of AM radio receivers and the term "modulated-AM-signal path" is thus meant to include the signal path in the RF section of a tuned radio frequency (TRF) receiver as well as to the signal path through the tandem combination in a superheterodyne receiver of the RF, mixer and IF sections. Also, the term "blanking" as applied to a signal path is used broadly herein to mean blocking the signal flow along the signal path, such as by interrupting the signal path, shorting the signal path or removing the electrical energizing source from a stage through which the signal path is routed.
The suppression of impulse noise in AM receivers has been accomplished by clipping off any impulse at the antenna that is greater than the amplitude modulation at the moment. This and other clipping circuits represent the simplest and least expensive AM noise blankers. Such noise clipping circuits do not eliminate the noise but just reduce its amplitude.
Another noise blanking system senses the noise in the RF or IF sections of a superheterodyne receiver and blanks the audio signal path. This system must employ a long blanking time that results in a thump sound from the receiver. This disadvantage is explained by the fact that nothing is done to protect the receiver RF and IF sections from overload and the pulse is stretched to a value equivalent to at least the period corresponding to the entire selectivity (bandwidth) of the receiver. As the noise impulse energy rises, the corresponding audio noise pulse is stretched more and more since the gain of the receiver will amplify the start and finish point of the initially stretched pulse in the IF filter farther and farther into IF amplifier saturation. This produces a long and variable length pulse. A low energy noise impulse is stretched to a minimum length, e.g. 150 microseconds, in the audio section determined by the IF passband, e.g. 452 KHz to 458 KHz. The audio noise pulse contains audio frequency components that are lower than the broadest audio frequency response that is determined by the IF passband, and thus at higher impulse energy levels, the audio noise pulse is even longer, e.g. 500 to 1000 microseconds. Also for high impulse noise the receiver AGC may be activated to the point that the desired signal is heavily attenuated.
A more complex but more effective and widely used noise blanking system in AM radios is one that senses the impulse noise in an early portion of the IF section of the receiver and blanks the signal path downstream at a point in the AM-modulated-signal path, e.g. in a later portion of the RF section of the radio. Such a system was first described by James J. Lamb in the paper entitled "A Noise Silencing I.F. Circuit for Superhet Receivers", QST, February 1936, pp. 11, 12, 13, 14, 38, 90, 92, 106, 108, 110 and 112, and in his patent U.S. Pat. No. 2,101,549 issued Dec. 7, 1937. Subsequently, a modified Lamb noise suppressor system senses the presence of a noise pulse in the RF section and blanks downstream in either the RF section of IF sections of the receiver. However, as is further explained below, this kind of noise suppressor system operative in the RF or IF sections of the receiver results in a bop sound at each incidence of a noise impulse. That sound is more pronounced in high fidelity AM radio receivers such as the recently introduced stereo AM receivers wherein such disturbances are even more objectionable.
It is therefore an object of the present invention to provide a noise blanking system that upon being connected to an AM radio receiver more effectively suppresses impulse noise therein.